ASM will drop a disk from a disk group if the disk stays offline for longer than the disk repair time. If the disk was taken offline because of an intermittent problem, for example a network problem, then you can re-add such disk to the disk group. Force option must be used for re-adding such disk because it already contains ASM metadata.
Example of re-adding a regular disk:
[grid@rac1 ~] $ flashgrid-dg add-disks -G MYDG -d /dev/flashgrid/rac2.xvdbb -f
Example of re-adding a quorum disk:
[grid@rac1 ~] $ flashgrid-dg add-disks -G MYDG -q /dev/flashgrid/racq.xvdba -f